Blender object being imported in Unity is invisible
Since your plane is in $XY$ in blender, it's actually in $XZ$ in unity if you exported by default values, so you can't see it by its side.
Try change to 3d view and see if this is the issue
Also, it could be mesh normals which determines front faces. Blender renders both faces, front and back but in unity its just front by default, but in this case I don't think it's the problem.